The invention relates to rotary drill bits for use in drilling or coring deep holes in subsurface formations.
In particular, the invention is applicable to rotary drill bits of the kind comprising a bit body having an external surface on which are mounted a plurality of cutting elements for cutting or abrading the formation, and an inner passage for supplying drilling fluid to one or more nozzles at the external surface of the bit. The nozzles are so located at the surface of the bit body that drilling fluid emerging from the nozzles flows past the cutting elements, during drilling, so as to cool and/or clean them.
Although not essential to the present invention, the cutting elements may be in the form of so-called `preform` cutting elements, being in the shape of a tablet, usually circular, having a hard cutting face formed of polycrystalline diamond or other superhard material.
In one commonly used method of making rotary drill bits of the above-mentioned type, the bit body is formed by a power metallurgy process. In this process a hollow mould is first formed, for example from graphite, in the configuration of the bit body or a part thereof. The mould is packed with powdered material, such as tungsten carbide, which is then infiltrated with a metal alloy, such as a copy alloy, in a furnace so as to form a hard matrix. (The term `matrix` will be used herein to refer to the whole solid metallic material which results from the above process, i.e. tungsten carbide powder surrounded by solidified infiltration alloy. This is the term commonly used for such material in the drill bit industry, notwithstanding the fact that, in strict metallurgical terms, it is the infiltration alloy alone which forms a matrix, in which the tunsten carbide particles are embedded.)
If the cutting elements are of a kind which are not thermally stable at the infiltration temperature, dummy formers are normally mounted on the interior surface of the mould so as to define on the finished bit body locations where cutting elements may be subsequently mounted. Alternatively, where thermally stable cutting elements are employed, such elements may themselves be located on the interior surface of the mould so as to become mounted on the bit body during its formation.
Although the aforementioned nozzles for supplying drilling fluid to the surface of the bit body may be formed by simple holes in the matrix material communicating with the inner passage of the bit body, it is preferable for each nozzle to be a separately formed assembly which is mounted in the bit body. This enables the nozzle aperture to be accurately dimensioned and also allows the nozzle assembly to be formed from hard, erosion-resistant material or faced with such material.
When bit bodies were first manufactured from matrix, using the above-described powder metallurgy process, it was common practice for the separately formed nozzle to be permanently embedded in the bit body during formation thereof. The nozzles would be mounted at the desired locations on the interior surface of the mould, and the powder material would be packed around the nozzles before infiltration. The disadvantage of this method was that since the nozzles were permanently mounted in the bit body the diameter of the nozzle aperture was fixed once the bit had been manufactured. However, there are many different factors which determine what size of nozzle aperture will give the best performance during drilling. Accordingly, it became desirable to mount the nozzles removably in the bit body so that the appropriate size of nozzle might be selected and fitted according to the particular drilling conditions. In order to achieve this, externally threaded nozzle assemblies have been provided, which screw into internally threaded sockets provided in the bit body. Since, in order to provide the required erosion resistance, the nozzles are often formed from tungsten carbide or similar hard material which is difficult to machine, the external thread for the nozzle has usually been provided on a steel sleeve which is brazed to the carbide of the nozzle.
With conventional matrix bits, however, it is difficult simply to machine an internal screw thread within a socket in the bit body, due to the hardness of the matrix material. Accordingly, it has hitherto been the practice, in order to provide replaceable nozzles in matrix bits, to mount within the matrix an internally threaded steel sleeve into which the nozzle may subsequently be screwed. Such arrangement has the disadvantage, however, that it involves several manufacturing steps and is therefore costly. Also, the necessity of providing a steel sleeve means that the effective overall diameter of each nozzle assembly is greater than the diameter of the nozzle itself and this imposes limitations on how closely nozzles may be mounted in relation to one another and to the cutting elements on the bit body and this, in turn, imposes undesirable restrictions on the design of the bit body as a whole.
If the threaded steel sleeve is embedded in the matrix during the formation of the bit body, problems may arise due to oxidisation of the sleeve and/or fouling of its threads by matrix powder. On the other hand, if the sleeve is brazed into a socket in the matrix after the matrix has been formed, there is always the risk that, occasionally, a brazed joint will be imperfect and liable to allow leakage. Such imperfect brazed joints may be difficult to detect during the manufacturing process. If leakage does occur, the steel sleeve becomes subject to erosion at both ends, and this can, in time, even cause the sleeve to become detached from the bit body.
It is also usually necessary to provide an O-ring seal between the nozzle assembly and the steel sleeve. Normally, such a seal will prevent any leakage of drilling fluid around the nozzle assembly. However, should leakage pass the O-ring occur for any reason, such leakage will begin to erode the steel around the O-ring, so that the leakage, once begun, will rapidly get worse.
The present invention sets out to provide a rotary drill bit, and a method of manufacturing such a bit, in which the above-mentioned disadvantages may be reduced or overcome.